<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
        const y = parseInt(prompt('请输入年份'))
        if (y % 100 == 0) {
            if (y % 400 == 0) {
                alert(y + '是闰年')
            } else {
                alert(y + '不是闰年')
            }
        } else {
            if ( y % 4 == 0) {
                alert(y + '是闰年')
            } else {
                alert(y + '不是闰年')
            }
        }
    </script>
</body>
</html>